MOSS, BEECH TREES, AND
STEMFLOW: INTEGRATED SCIENCE
By Michelle Frasco, Pinkston Street Elementary School and
Christopher Dobson, Grand Valley State University
A Mossy Misconception
Have you ever been lost in the woods? Have
you heard that, “moss grows on the north
side of trees,” from someone presumably
imparting navigational wisdom? Is there
anything to this popular old adage? Well…
yes and no. Wandering through the woods,
you will find that moss does not grow exclusively on the north side of trees and that
this common misconception may not lead
you home. However, you might observe that
moss can be more abundant on the north
side of trees, at least in the Northern Hemisphere. Why does this particular pattern
occur in nature?
You may already know the answer, but in
this article we uncover an intriguing exception to the rule, something entirely unexpected! We describe our personal discovery
of stemflow and how it can dramatically
influence where moss is found on beech
trees. What is “stemflow” you ask, and what
does it have to do with moss? And what’s
the deal with beech trees? Good questions,
read on. Our narrative has all the twists and
turns of any good mystery, providing a review of relevant scientific concepts along the
way. Our primary goal, however, is to use
the relationship among moss, beech trees,
and stemflow as an opportunity to discuss
integrated science, specifically what it is,
why it’s important, and how to implement it
in your classroom.
Life Science, Physical
Science, and Earth
Science…Oh My!
Like all plants, moss must reproduce and
disperse its offspring. It does so via spores
that result from the fertilization of eggs
by flagellated sperm. This helps explain
Each day celestial noon occurs
when the sun has reached its
highest point in the sky and is
halfway between sunrise and
sunset. In comparison to “clock
time,” celestial noon varies and
depends on a location’s longitude,
and whether it is day-light savings
or standard time.
why moss is found in wet environments.
“Swimming” sperm, produced in the male
antheridium, require a film of water to reach
an egg in the female archegonium (Figure
1). Following fertilization, a spore-containing
capsule, the sporangium, develops and
releases spores. The spores germinate and
grow into moss plants when conditions are
favorable, which includes available moisture.
Patterns that occur in nature have causal
explanations. In the case of moss, the role
of water in its life cycle (sexual reproduction
and spore germination) accounts for the
plant’s distribution and abundance in nature
(life science concepts).
You may also be aware that moss commonly grows on trees in an epiphytic relationship. The moss is nonparasitic to the tree,
obtaining its nutrients from air and rainfall.
The particular side of a trunk that moss
grows on can be influenced by sunlight.
After light from the sun has traveled through
Earth’s atmosphere and reached an object
on the ground, it is blocked by the object
itself, leaving the other side of the object
shaded. Shade reduces the evaporation of
water due to sunlight, making the shaded
side of a tree trunk somewhat wetter (physical science concepts). u
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27
To understand where shade occurs one
must comprehend the path of the sun
through the sky. From the center of the
Lower Peninsula (St. Louis, Michigan), the
sun is farthest north at celestial noon on the
summer solstice, 70.5º above the southern
horizon. The sun is farthest south at celestial
noon on the winter solstice, 23.5º above the
southern horizon. Over the course of a year,
the sun’s altitude on celestial noon is always
located between these points, with sunlight
directly striking the south side of objects,
including trees (Figure 2). This year-round
southern exposure results in a slightly cooler
and wetter microenvironment on the north
side of trees (earth science concepts).
Assimilating knowledge of the life, physical, and earth science concepts described
in the last several paragraphs, it makes
sense to predict that moss should be more
Figure 1. Fertilization and spore dispersal in moss.
Figure 2. Altitude of sun on summer and winter solstices results in yearround southern exposure.
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• MSTA Journal • Spring 2010
abundant on the north side of trees, at least
in Michigan and elsewhere in the Northern
Hemisphere.
Making Predictions...
(a.k.a. Hypothesis Testing)
Wandering through a wooded ravine system one day, we noticed a large American
beech tree (Fagus grandifolia) with a
healthy population of moss growing on its
trunk. We suspected, however, and then
confirmed with a compass, that there was
noticeably more moss on the south side of
the trunk. We were on a south-facing slope
with southern exposure and expected moss
to be more abundant on the north side of
the trunk. More moss on the south side of
this particular tree contradicted our prediction, based on our understanding of the
life, physical, and earth science concepts
discussed above. That’s how it works in science: a hypothesis is a possible explanation
for some observed phenomenon that leads
to a specific prediction, which allows you to
test the hypothesis. If the prediction comes
true the hypothesis is supported. If not, the
hypothesis should be rejected. Our prediction that moss would be more abundant on
the north side of the tree did not come true
and forced us to question our hypothesis.
Remember, we made our prediction based
on the hypothesis that the north side is
shaded (wetter), providing a more favorable
environment for moss to complete its life
cycle (reproduction and germination). Finding more moss on the south side of the tree
was puzzling, to say the least!
Reluctant to completely reject our hypothesis based on one trouble-making beech
tree, we knew we needed a larger sample
size. As always in science, the results
needed to be repeatable. Could we find
additional trees with moss more abundant
on the south side of their trunks? We also
began to wonder if the fact that this tree was
growing on a relatively steep slope, leaning
slightly downslope, could be impacting moss
growth. So we decided to conduct an investigation of moss abundance on tree trunks
on north- and south-facing slopes, focusing
on our primary suspect, the American beech
(Figure 3). We also thought it prudent to pay
attention to whether or not trees tended to
lean downslope, and if so, how much. We
decided to estimate the degree of lean of
each tree away from vertical, if any, using a
weight attached to a protractor with string
(plumb bob method). u
Figure 3. Beech trees on north- and south-facing slopes. slopes.
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We suddenly realized that we were testing
two competing hypotheses, each leading
to different predictions. If a more shaded
environment on the north side of trees is
most influential in determining moss abundance (shade hypothesis), we expected to
find more moss on the north side of trees on
both north- and south-facing slopes. If trees
tend to lean downslope, and that somehow
favors moss growth on the downslope side
of trees (downslope hypothesis), we predicted that we would find more moss on the
north side of trees on north-facing slopes
and on the south side of trees on southfacing slopes. Remember, hypotheses lead
to specific predictions, and these predictions
allow you to test the hypotheses. Hot dog,
we were hypothesis-testing!
You may already see this coming, but
we found significantly more moss on the
downslope side of trees on both slopes.
Again, because this is worth restating, moss
was consistently more abundant on the
south side of trunks on south-facing slopes.
We also found that trees on both slopes did
tend to lean downslope, an average of about
six degrees from vertical. These findings
supported our downslope hypothesis and
forced us to reject our shade hypothesis, or
at least acknowledge that it needed modification (more on this later). This is definitely
not what we had expected, moss preferring
the south side of so many trees! We had a
genuine mystery on our hands. Fortunately,
we found our mystery-solving clue not long
thereafter. It happened one cold, rainy day.
That’s when we saw it—stemflow!
Stemflow Runs Down
Lichen Lane
It was raining fairly hard, and we were
huddled underneath a large beech recording
moss abundance and the degree of tree lean
away from vertical. As with most trees on the
slopes we investigated, this particular beech
was leaning noticeably downslope. To our
amazement, we saw a gushing “stream” of
water funneling down the downslope side of
the trunk, leading straight to a healthy patch
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• MSTA Journal • Spring 2010
of moss at the base of the trunk. We later
came to learn that this phenomenon is called
stemflow. Stemflow occurs when it’s raining; water accumulates in the upper leaves
and branches of trees and is channeled
downward toward the trunk. It is especially
pronounced in trees with smooth bark like
beech. On flat terrain trees tend to grow relatively vertically. This results in stemflow down
one or more sides of the trunk, depending
on various ridges and grooves in the bark,
as well as the particular orientation of major
branches feeding water to the trunk. What
we discovered that day is that when a beech
tree is leaning, stemflow runs along the
downslope side of its trunk (Figure 4). We
quickly hypothesized that the reason moss
is more abundant on the south side of beech
on south-facing slopes is that the trees tend
to lean downslope, directing stemflow to that
side of their trunks. More water, more moss.
Solving our mystery of why moss would be
more abundant on the south side of so many
trees seemed close at hand.
Of course, there was that whole “repeatable”
thing we have to concern ourselves with in
science. Would we see stemflow occurring
primarily on the downslope side of other
beech trunks on sloped terrain? One difficulty in answering this question was that it
required us to be out with our trees in rainy
weather, not always easy to accomplish.
That’s when we made our second casecracking discovery. As we watched water
funneling down the trunk of that large beech
tree, we noticed an abundance of lichen
growing right along the path of stemflow.
Lichen is a symbiotic relationship between
a fungus and algae and/or bacteria. Like
moss, lichen is a nonparasitic epiphyte,
getting all its nourishment from air and
rainfall. Naturally, lichen would benefit from
stemflow in a similar fashion to moss. More
water, more lichen. Then it dawned on
us; we could use lichen growth on a trunk
to estimate the path of stemflow on days
without rain (Figure 5). Using this indirect
measure of stemflow along trunks, we were
able to answer our remaining question about
its location on slopes. Stemflow did occur
primarily on the downslope side of beech
trees on sloped terrain. This result correlates
with moss abundance. Both stemflow and
moss occurred more frequently on the north
side of trees on north-facing slopes, and
on the south side of trees on south-facing
slopes. Mystery solved, case closed! Well,
almost. u
Figure 4. Stemflow on a vertically growing tree & on one leaning
downslope.
Figure 5. Lichen growth indicating stemflow down trunk.
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Identifying and
Explaining Patterns in
Nature
Scientists look for patterns in nature and
then try to provide explanations for those
patterns. In this article we have identified
an intriguing pattern of preferential moss
growth on the south side of beech trees on
a south-facing slope. Our original hypothesis was that moss needs water to complete its life cycle and that the north side of
trees is more shaded, providing a moister,
more favorable environment. This led to the
prediction that moss should be more abundant on the north side of trees. Because we
found more moss on the south side of our
beech trees, our hypothesis requires modification. In this case, we need not reject our
hypothesis entirely. Rather, we must revise
it, taking stemflow into account. In science,
the iterative process of hypothesis testing
and modification advances our understanding and improves our explanations of the
patterns we observe in nature.
In general, moss may be more abundant on
the north side of trees, although exceptions
obviously occur. Moss has no compass; it
simply thrives where moisture is available.
Any circumstance that provides adequate
water to a specific portion of the trunk, such
as tree lean or a particular branch orientation, will foster moss growth there. Because
beech have smooth bark, stemflow is especially pronounced and appears to trump cardinal direction, north or south, in determining
where moss is most abundant. Other trees
commonly found in Michigan forests, such
as sugar maple (Acer saccharum), possess
bark that is deeply furrowed, or grooved. This
seems to reduce the shift of stemflow from
one side of a trunk to the other. Stemflow
on leaning maple trees may not end up on
the downslope side of trunks as readily as
it does on beech. We have not investigated
this, but would be interested to hear what
you and your students find should you do so.
In any case, our investigation does highlight
how hypotheses often warrant modification,
rather than complete abandonment.
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• MSTA Journal • Spring 2010
Throughout this article, we have framed our
explanation for the distribution of moss by
considering relevant concepts in the life,
physical, and earth sciences. We continue to
do so by exploring additional interactions and
circumstances that lead to the formation and
path of stemflow along the downslope side of
trunks. During a rain, the force of gravity acts
on water droplets that accumulate on leaves
and branches in a tree. Rather than dripping
to the ground, much of the water sheets
down increasingly larger branches, merging
like the tributaries of a river. On a leaning
beech, stemflow eventually makes its way
to the underside of the trunk, the downslope
side, again because of gravity. Water (H2O)
also happens to be a polar molecule, and
hydrogen bonding between individual molecules results in cohesion and surface tension
as it streams down the tree. And although
the bark is wettable, absorbing some water
through adhesion to hydrophilic substances
such as cellulose, the bark is mostly impermeable to water as a result of hydrophobic
interactions with the fatty substance suberin.
Together, gravity and interactions due to the
polar nature of H2O produce a steady stream
of water feeding the downslope side of
trunks (physical science concepts). In addition, tree lean on the slopes we investigated
is due to creep, the slow, gradual downslope
movement of soil (earth science concept). A
tree tilts downslope at its base as the ground
below it is subjected to creep, and responds
phototropically by bending and growing
straight upwards toward the sun (life science
concept). Creep is a type of mass wasting,
the movement of surface land masses due
to…interestingly enough…gravity. Other
mass wasting processes, such as landslides,
also tilt trees, potentially impacting stemflow
as in our beech trees.
Some of these concepts may be unfamiliar
to you, but including them illustrates how
truly understanding a particular pattern in
nature, such as moss growth on trees, can
involve several levels of complexity. We
realize that, depending on grade level, some
of the concepts presented may be inappro-
priate or too detailed for your students. We
are also cautious to generalize the results of
our investigation, from beech on some local
north- and south-facing slopes, to other tree
species and forested regions. Our findings
do, however, call into question the popular
wisdom of using moss on the north side of
trees as a navigational tool. Clearly, there
is much involved in explaining the distribution and abundance of moss on any given
tree, and this explanation repeatedly and
indiscriminately crosses boundaries of the
traditional science disciplines (life, physical,
& earth sciences).
iNteGraTEd SciEnCe
Now we use our investigation of moss,
beech trees, and stemflow as a springboard
for discussion of integrated science, specifically what it is, why it’s important, and how
to implement it in your classroom. Like the
inconsistent capitalization in our section
heading above, interpretations of integrated
science among educators vary. A general
lack of consensus on the definition and
conceptualization of integration permeates
science education. So what is integrated
science? There is integration of mathematics in science instruction and integration
of science across the curriculum with the
social sciences and humanities (e.g., a
writing assignment on the history of the
microscope). Integration also occurs within
a particular branch of science, such as
biology, through the simultaneous learning
of distinct but related concepts (e.g., genetics, evolution, and ecology). These types
of integration, however, do not constitute
“integrated science” in its intended sense,
which requires the combination of concepts
from multiple science disciplines to understand some topic or issue.
Blum (1991) claimed that integration among
the branches of science occurs in two ways:
1) When the unity of knowledge and procedures among them are emphasized, as
intended by the National Science Education
Standards (NRC 1996) unifying concepts
and processes (e.g., “systems, order, and
organization” or “evidence, models, and
explanation”); and 2) When the sciences are
taught in an integrated way for pedagogical
reasons; concepts from an additional discipline are needed to understand a problem
being studied within a particular branch of
science. Venville et. al (2002) proposed that
integration in this second sense falls along
a continuum from multidisciplinary to interdisciplinary, to transdisciplinary, with each
additional label signifying a reduction in
compartmentalization between disciplines.
The Biological Sciences Curriculum Study
(BSCS 2000) suggested thinking of integration as the combination of parts to make a
whole. They described integrated science
as requiring students to make meaningful
and coherent connections among disciplinespecific concepts that connect logically and
obviously:
For coherence to prevail, the physics,
chemistry, and biology must be woven
into a discernable whole that draws
on but transcends the coherence that
characterizes each of the individual
disciplines.
Connections among the sciences must be
evident, purposeful, and critical for comprehension of some larger concept. Learning
science in this manner blurs boundaries
between the traditional disciplines and,
in our opinion, is the intended meaning of
integrated science.
Consider the middle school content statements below from the Grade Level Content
Expectations (MDE 2009) that are related to
understanding the growth of moss on trees
(content statements in italics and relevance
to moss abundance in parentheses).
Life Science:
• L.EV.M.1 – Species Adaptation and
Survival (moss adapted to particular
environments)
• L.EC.M.3 – Biotic and Abiotic Factors (water availability impacts moss abundance) u
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• L.OL.M.6 – Photosynthesis (moss uses
sunlight to make sugar from H2O and CO2)
• L.HE.M.2 – Reproduction (moss life
cycle: sexual reproduction and spore
germination)
Physical Science:
• P.FM.M.2 – Force Interactions (effect of
gravity on stemflow)
• P.EN.M.4 – Energy Transfer (solar radiation carries energy from the sun to Earth)
• P.CM.M.1 – Changes in State (water
changes from liquid to gas during evaporation)
• P.EN.M.3 – Waves and Energy (sunlight
increases evaporation of H2O on trunks)
Earth Science:
• E.ES.M.6 – Seasons (southern exposure
makes north side of trees wetter)
• E.SE.M.1 – Soil (creep and other mass
wasting processes cause trees to lean)
• E.ES.M.1 – Solar Energy (sun drives
water cycle through evaporation of H2O)
• E.ES.M.8 – Water Cycle (stemflow down
trees contributes to water cycle)
Why is integration important? An integrated
approach in science instruction is consistent with the goal of increasing scientific
literacy among students (AAAS 1989 and
NRC 1996). Bybee (1997) clarified scientific
literacy as including an understanding of
science content, scientific inquiry and the
nature of science, and the interrelationships
of science, technology, and society. In
addition to helping students learn concepts,
adopting an integrated approach facilitates
a more sophisticated perception of science
and makes learning personally relevant
through the context of social or environmental issues. For example, evaluating global
climate change requires an integration of
energy, atmosphere, and impacts on plant
and animal life (physical, earth, & life science concepts), an understanding of the
methods and uncertainty in science, and
social implications of scientific findings.
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• MSTA Journal • Spring 2010
How can you implement integrated science
in your classroom? In our opinion, a thematic approach like global climate change is
not required. Although social or environmental issues lend themselves to integration,
many natural phenomena that your students
could study are inherently integrated. Take,
for instance, the smaller and more specific
question about moss growth on beech trees.
Our investigation led us on a journey that
repeatedly traversed discipline boundaries, requiring an exploration of concepts
throughout the life, physical, and earth sciences. An integrated examination of these
discipline-specific concepts was necessary
to understand moss abundance. None of
the concepts alone sufficiently explains the
pattern of moss growth we observed.
A working knowledge of Michigan science
standards (MDE 2006 & 2009), however, is
a must. We suggest viewing the standards
as less prescriptive, and more as guidelines.
You need to be familiar enough with them to
recognize which are addressed in any given
lesson or unit. You could choose to first
review standards throughout the life, physical, and earth sciences in order to select an
integrating topic to investigate. For example,
you might choose to highlight conservation
of mass (physical science), photosynthesis
(life science), and atmospheric composition
(earth science) in exploring how primitive
photosynthetic bacteria changed Earth’s
early environment through millennia of
oxygen production. Alternatively, you could
identify standards from among the disciplines that are relevant to some topic which
you or your students have already elected
to study. As an example, imagine that your
students become interested in learning
about biofuels. You could peruse the standards to determine which life, physical, and
earth science concepts can be addressed in
learning about this topic. Either way, it’s crucial to help students explicitly recognize the
integration of discipline-specific concepts
needed to understand our world. In addition, a knowledge of standards outside your
current grade level will allow you to review
content previously learned by students and
even introduce concepts they will learn in
future grades.
Adopting an integrated approach does not
necessarily require major changes to your
current curriculum. It may, however, call for
thinking about your curriculum differently.
We suggest viewing your curriculum along
a continuum from complete integration,
with some overarching theme like global
climate change, to a few small-scale, integrated lessons such as moss distribution
and abundance. Think baby steps! You do
need to carefully consider the integration of
content in your lessons, making connections
between the sciences evident, purposeful,
and critical to understanding some larger
concept. But don’t feel obligated to incorporate concepts from each discipline in every
lesson; connections should be meaningful
and coherent. Finally, we advocate learning
through inquiry as an inherently integrated
approach, because students explore concepts from multiple disciplines to answer the
questions of their investigations.
In this article, we shared our discovery of the
fascinating relationship among moss, beech
trees, and stemflow. The excitement we felt
in uncovering the effect of stemflow on moss
growth resonates as a central feature of
scientific inquiry, one we want our students
to experience firsthand. We also wanted to
highlight the use of hypothesis testing and
modification in science for explaining patterns observed in nature. Our main objective, however, was to use our investigation of
moss to illustrate the meaning of integration
as it relates to the concept of integrated
science. We intend no authoritative stance
on the subject; rather, we wanted to present our interpretation of integration as we
continue to explore and struggle with how
best to accomplish it in our classrooms. We
hope to have stimulated interest, potentially
initiating a dialogue among Michigan educators about the meaning and importance of
integration in science education. At the very
least, you should think twice before using
moss growth on trees to find your way out of
the woods!
Acknowledgments
We would like to thank students Marisol
Arana and Amanda Deaton for their help in
data collection and analysis, Andrew Duryea
for Figure 1, and Nicole Hammer for Figure
5. We also appreciate expertise provided on
stemflow and soil creep by Dr. Neil MacDonald and Dr. Pablo Llerandi-Román (Grand
Valley State University), and Dr. Delphis
Levia (Research University of Delaware).
References
American Association for the Advancement of
Science. 1989. Science for all Americans: A
Project 2061 Report. Washington, DC: AAAS.
Biological Science Curriculum Study. 2000. Making
Sense of Integrated Science: A Guide for High
Schools. Colorado Springs, CO: BSCS.
Blum, A. 1991. Integrated science studies. The
International Encyclopaedia of Curriculum, ed.
Lewy, A., New York: Pergmon Press.
Bybee, R. 1997. Achieving Scientific Literacy:
From Purposes to Practices. Portsmouth, NH:
Heinemann.
Michigan Department of Education. 2009. K-7
Science Grade Level Content Expectations.
Michigan Department of Education. 2006. Michigan
High School Science Content Expectations.
National Research Council. 1996. National Science
Education Standards. Washington, DC: National
Academy Press.
Venville, G., J. Wallace, L. Rennie, and J. Malone.
2002. Curriculum integration: Eroding the high
ground of science as a school subject. Studies in
Science Education, 37: 43-84.
About the Authors
• Michelle Frasco is a graduate of the Integrated Science Program at Grand Valley State University and currently teaches 4th grade math at Pinkston Street Elementary School in Henderson,
North Carolina.
• Christopher Dobson ([email protected]) is an associate professor in the Biology Department
and member of the Integrated Science Program at Grand Valley State University.
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